1. Field of the Invention
The present invention relates to the use of fluid retarding, ion conducting material used to isolate electrolyte products from the region of electronic sensing of particles.
2. Discussion of the Prior Art
Various impedance or phase based particle sensing devices exist in the prior art for studying the physical properties of microscopic particles, such as biological cells carried in a liquid suspension, as illustrated by the pioneer U.S. Pat. No. 2,656,508, "Means for Counting Particles Suspended in a Fluid" W. H. Coulter, Oct. 20, 1953; and U.S. Pat. No. 4,014,611, "Aperture Module for Use in Particle Testing Apparatus", Simpson et al., Mar. 29, 1977. A well known "Coulter principle" of operation is referred to with particularity in these patents. Generally, these Coulter devices include two fluid vessels or chambers, each containing a conductive electrolyte solution. At least two electrodes having opposite polarity are immersed in the electrolyte solution, with each fluid compartment having one of the electrodes disposed therein. A sample of the electrolyte solution, having the particles suspended therein, is passed through a constricted fluid path or orifice interposed between the two fluid compartments. Although this constricted path can take different forms, in each device such path defines a sensing zone wherein the presence or absence of a particle in the constricted path gives rise to a detectable change in electrical characteristics of the path. For example, relatively poorly conductive biological cells passing through this constricted path displace a volume of electrolyte solution equal to the cell volume, causing a voltage drop by increasing the path impedance. To put it another way, the resistance between the two electrodes which are separated by the constricted path is increased by the cell presence. The resistance pulses defined by the voltage drops are used for particle counting and particle volume determination.
Modification of the above described prior art sensing scheme has led to the development of particle sorters, wherein the selective resistance pulses provided by constricted path activates the sorter to charge individually isolated droplets containing the activating cells. The charged droplets are deflected from the main stream by a static electric field into a collecting vessel. Typically, the prior art sorters include a first and a second sheath flow, with the second sheath flow being introduced below the orifice. A downstream return electrode is mounted in the second sheath. Consequently, the downstream particles are exposed to undesirable electrode products produced by the return electrode. An illustrative particle sorter has been sold by Coulter Electronics, Inc. of Hialeah, Florida.
In order to sense the impedance changes, it is necessary to have a current flow between at least two electrodes in the case of DC currents. The current flow is due to ions which proceed toward the oppositely charged electrode. However, there are several inherent problems brought about by this electrolysis process, which next will be discussed.
Almost all electrolyte solutions create unwanted gas at the electrodes. For instance, the electrolyte sodium chloride in solution (saline solution) forms oxygen, chlorine and hydrogen gases which take the form of gas bubbles, such bubbles frequently create noise in the impedance sensing device as such bubbles travel through the constricted path. At the same time, other undesirable electrolyte products are produced. For example, in the case of sodium chloride, hypochlorite is formed by the chlorine gas acting with the water, and can kill or damage biological cells.
Electrolysis normally changes the pH of the solution, such as where hydrogen ions form hydrogen and thereby make the solution more basic. Cells generally are viable only in a specific pH range, and such pH changes can even kill the cells. Moreover, the user may be operating the impedance sensing device based on assumed cell environment conditions. However, a change in pH, and therefore a change in cell environment, can lead to different physical properties of the cell, such as changes in the cell membrane. These different physical properties can lead to a change in cell volume; hence, a change in the detected resistance. Moreover, the electrodes can be fouled by the presence of various substances, including proteins.
Accordingly, it readily can be seen that there has been a long recognized need in the art of cytology to prevent the electrolyte products from interfering with the impedance sensing device and sorting processes.
In the case of simple impedance based cell sorters, such as the previously cited sorter, or more simply where the cells are to be collected, it is necessary to minimize the volume of liquid beneath the orifice. First, this minimized volume is desirable for the purpose of providing fidelity of collection, and secondly, not impeding fluid flow. Since the power electrodes must be of a finite size, it is necessary to position the downstream electrode remotely from the orifice.
The use of frits, gels and membranes in chemical art areas is well known. For instance, electrophoresis involves the movement of charged, dispersed particles in a colloidal system toward electrodes that have opposite charges, such process normally being used to separate molecular species, such as proteins which differ by charge or charge and shape. In order to separate properly the molecular species, it is desirable not to have bubbles which create fluid turbulences and changes in pH, which effect the mobility (velocity) of the species being separated. In short, a constant chemical composition of the solution employed to perform these separating tests is required. Consequently, fruits and other such means are used to separate the volume holding the electrodes from the volume in which separation occurs. However, there is no electronic sensing of individual particles in the electrophoresis process.
In prior art pH and other ion sensing meters, frits, gels and like means are used to protect and separate and maintain the precisely defined chemical milieu that is disposed around the internal electrode of the reference electrode from the solution being measured by the pH meter. However, in that there is a minimal amount of current in the pH meters, electrolyte products are of negligible consequence. The current in a pH meter is of the order of one billionith of that in a standard particle sensing transducer. Other chemical apparatuses, such as polargraphs and electrolylic half cells, use various conducting gels and frits. However, none of these processes involve impedance sensing of particles.